1. Field of the Invention
The present invention relates to programmable resistance memory devices, including phase change memory devices, and to refresh techniques associated with maintaining data values stored in such devices.
2. Description of Related Art
Phase change based memory materials, like chalcogenide based materials and similar materials, can be caused to change phase in active regions of a memory element between an amorphous phase and a crystalline phase by application of electrical current at levels suitable for implementation in integrated circuits. The amorphous phase is characterized by higher electrical resistivity than the crystalline phase, and the difference in resistance can be readily sensed to indicate data.
In order to achieve high density memory, smaller memory cell structures are needed. However, attempts to reduce the size of the phase change material element and/or the electrodes can result in data retention issues. For example, in a memory cell programmed to a high resistance state having amorphous phase material in an active region of the memory element, environmental conditions can result in drift in the resistance due to re-crystallization of small portions of the active region. When the resistance drifts out of the sensing margin specified for the programmed data value, then the data is lost. Similar issues can arise in other types of programmable resistance memory materials.
Thus, methods to improve data retention for phase change memory devices based on refresh operations like those encountered in dynamic random access memory have been investigated. See, United States Patent Application Publication No. US 2006/0158948 A1, by Fuji.
However, the need remains for efficient technologies to detect cells that need refresh (called margin cells herein), and to refresh such cells with correct data values.